LEDs combine design flexibility with practically robust circuitry that enables automotive designers to produce stunning headlamp designs with exceptional longevity and performance. Keith Szolusha, Senior Design Leader for Linear Driver Products at Linear Technology, is now a member of Analog Devices. He explains how automotive designers can incorporate LEDs into the lighting field because they can be arranged in unique, eye-catching designs - to help differentiate Old and new models, or high-end economics. LED matrix dimmer demonstration circuit operates as a Linduino® shield There is no doubt that automotive LED lighting has arrived but it has not reached its full potential. Future models will use more LED lights, including new shapes and colors, as well as more control over a single LED. A simple LED string will allow the individual dimmed LED matrix to be controlled by a computer, enabling unlimited real-time mode control and animation. Taking the combination of the LT3965 matrix LED driver and the LT3797 boost and then dual step-down mode driver as an example, it will be shown how the advanced level of driver technology can make the next step in automotive lighting design easier. I 2 C control of 8 power switches with a single IC The basic LED headlamp design works with a uniform LED current, so the brightness is even. But this leaves a lot of LED potential on the desktop. The matrix lamp fully utilizes the inherent ability of the LED by controlling the brightness of each LED in the LED string. In theory, it is not difficult to handle a single LED in a matrix through a computer-controlled power switch, allowing individual LEDs to turn on or off, or PWM dimming to create unique patterns and functions. Each LED (or part of the LED) requires its own converter or its own shunt power switch. It is possible to build a matrix driver with serial communication functions using conventional driver/converter ICs, but once the LED matrix requires more than two or three switches, designing a discrete component solution becomes challenging, involving a matrix of components beyond The size of the LED matrix. The IC 8 switch matrix LED dimmer easily controls large or small LED arrays (up to 512 LEDs). Figure 1 shows the drivers on the demo circuit. Figure 1: The LED matrix dimmer demonstration circuit operates as a Linduino® shield. The demonstration circuit runs headlights, turns lights, taillights and decorative patterns, and can be evaluated through a graphical user interface via USB cable Its highly integrated design (Figure 2) minimizes the number of components. Individually addressable channels can be used to control the LED matrix in a variety of ways. Figure 2: Block diagram of a 60-V 8-switch LED matrix dimmer showing eight power NMOS shunt switches for brightness control, fault flags, and I2C serial communication interface LED Matrix Control Options: Each driver can control 8 dimming channels - 8 LEDs or 8 clusters - in a string of LEDs Eight channels control red, green, blue, and white light on two RGBW LED modules to adjust brightness or change the color of the dashboard or decorative lighting Multiple drivers can be individually addressed on a single communication bus to multiply a string by a large array One driver can control multiple LEDs per channel, or can combine channels to efficiently control the current of a single LED When used in conjunction with a suitable constant-current LED driver, the matrix dimmer LED driver allows the individual LEDs to run through the headlights, daytime running lights, brakes and taillights, side-curved lights, instrument panel displays and other decorative lighting control. Built-in automatic fault detection protects a single LED in the event of a fault and reports the fault to the microcontroller. The 60 V driver contains eight integrated 330mΩ power switches that can be connected to one or more LEDs. The power switch acts as a shunt device by turning off or dimming LEDs on specific channels. These switches create 8 individually controlled luminance channels (up to 256:1 dimming ratio) and fail-safe segments of 8 LED strings. When all eight power switches are on at the same time (all LEDs are off), it can handle a 500 mA string current. As described later in this article, the switches can be connected in parallel and run at 1A with four channels of LEDs. Regardless of the number of LEDs or the current, the LED string must be driven by a well-designed converter that has the bandwidth to handle the fast transients of the matrix dimmer. This article contains some reference designs. Boost, then dual buck mode to drive two strings, 16 LEDs, 500 mA with two matrix drivers The eight shunt power switches of the matrix LED driver control the brightness of the eight channel LEDs at 500 mA. The string voltage of the 8-LED matrix dimmer system can be between 0 and 26 V, depending on how many LEDs are on or off at a given time. The recommended converter topology for driving these LEDs is a 30 V step-down converter with high bandwidth and little or no output capacitance. This buck topology requires a 9-16 V automotive input "pre-boosted" to a 30 V rail where the buck regulator can operate. Triple Output LT3797 LED Controller Can Be Easily Used as a Single IC Solution for "Pre-Boost" and Buck Functions - Can Be Configured as a Boost Regulator on One Channel and then Configure the Step-Down LED Driver in Another On two channels. Each of the two buck LED drivers can drive a string of matrix dimmed LEDs. This topology has many advantages. Most notably, the circuit continues to operate optimally regardless of whether the LED string voltage is higher or lower than the battery voltage. Figure 3 shows the schematic of the demo board shown in Figure 1, which is a boosted, dual step-down mode matrix dimming headlamp system with 16 500 mA LEDs. Each LED can be individually controlled on/off or PWM dimmed to 1/256 brightness. The 350 kHz switching frequency is outside the AM band (good for EMI), and the 170 Hz PWM dimming frequency produced by the same 350 kHz clock is higher than the visible range. After the system is correctly synchronized, the matrix headlamps do not flicker. Figure 3: Matrix LED dimmer system with a boosted buck mode LED driver and two matrix dimmers that can drive 16 LEDs of a car battery at 500 mA. I2C serial communication controls the brightness of individual LEDs and checks for LED and channel failures The LT3797 buck mode converter is optimized for extremely fast transients with little output capacitance and proper compensation for control loops. These >30 kHz bandwidth converters can withstand fast LED transients because the LEDs turn on and off at any time and the PWM dims. The filter capacitor placed on the LED sense resistor replaces a pole in the control system and is lost for the fast transient performance of the matrix dimmer when the output capacitor is reduced or removed. The charge pump from the switching node is used to provide a voltage above the VLED+ voltage by more than 7 V to the VIN pin of the matrix LED driver to fully enhance the top channel NMOS when driving. Even if all eight shunt switches are turned on and the entire LED string is turned off, the low RD(ON) NMOS switch can still achieve high power operation without the IC becoming warm. In this case, the LED driver can keep the virtual output created by all eight shunt switches shorted without any problems and can quickly adjust the 500 mA with the next open LED. The demo circuit in FIG. 1 has the system shown in FIG. 3 and uses the attached IC microcontroller to operate the matrix headlamps through the Linduino® One demo circuit. The demo board in use operates as a Linduino large shield with serial codes up to 400 kHz and can generate different headlight patterns and interface with the graphical user interface (Figure 4). In the GUI shown in Figure 4, the LED brightness and fault protection features can be checked for open and shorted LEDs through the "All Channel Mode" and "Single Challenge Mode" commands and the "Fault Check" read and write commands. This demonstration circuit system can check flicker-free operation, fault protection and transient operation. The DC2218 can be plugged directly into a 12 V DC power supply and can be controlled from a personal computer running the GUI or reprogrammed via a simple USB connection. Figure 4: PC-Based Interface Allows Designers to Access LED Control and Monitoring 1 Matrix LED Driver Using Parallel Channels The LT3965 can be used to drive a matrix of 1A LED channels. It is easy to connect the power switches in parallel, so the two power switches divide the LED current into 1 A and each driver controls four 1 A channels. One way to use parallel power switches for higher currents is to run each inverter parallel switch only within 50% of the PWM period. With an NMOS power switch operating alternately for 1 A half the time, the effective heating is approximately equal to running 500 mA through the same NMOS at all times. Figure 5: 1 Matrix LED driver combines inverting shunt channels for higher current applications in high power LED headlamp systems Figure 5 shows a 1A matrix headlamp system that uses eight LEDs driven by two matrix LED drivers and another boosted buck mode IC. When PWM dimming, the IC uses a unique 1/8 cycle phase of 8 switches, as shown in Figure 6. In this 1 A-matrix system, the boost driver and buck mode LED driver channels are combined in parallel so that the paired channels are inverting, making 180° to each other; especially for channels 8 and 4, 7 and 3, 6 and 2 and 5 and 1 are paired. The parallel shunt alternates the flow, effectively doubles the PWM frequency, and has the advantage of diverting shunt current and heat. For this function to work properly, the maximum duty cycle of any single parallel power switch is 50%, because there are two inverting switches (split every 50% of the time) to turn the LED off 100% of the time in 50% of the time . Figure 6: 1/8 PWM flicker-free phase of eight matrix LED driver power switches limits transients during PWM dimming brightness control Each matrix LED diver controls the brightness of four 1 A LEDs driven by two 1 A buck mode LT3797 channels (from a boosted 20 V channel). This high power, powerful system can increase power by paralleling more channels in more LT3965 or higher current LEDs to power more LEDs. Two LEDs can be driven on each channel of 1 A and drive the power of this flexible headlamp system. Multiple LEDs per channel The matrix driver can support one to four LEDs per channel. Although it may be advantageous to individually control the fault protection or high resolution mode of each LED, it is not always necessary. Using more than one system per channel LED to reduce the number of dimmers in the matrix and enough to implement patterns or dimming requires some design. Headlights, lights, and taillight segments can have as many as four LEDs with the same brightness. Emergency LED lights can have a group of three and four LEDs flashing and using the same pattern wave. Figure 7: The flexible substrate LED driver can drive LED channels on separate LED strings and can be between 1 LED drive per channel. (The complete driver circuit is similar to Figure 3, but with only one IC, as shown) The circuit in Figure 7 demonstrates a two-channel LED system - it has the same number of LEDs as the circuit in Figure 3, but uses only a single matrix dimmer instead of two. When the IC command tells the LT3965 to turn on, turn off, or dim the channel, it will affect the two LEDs controlled by the shunt power switch of that channel. In order to maintain the IC's voltage limit, the 16 LEDs at 500mA still need to be split into two series-connected LED strings, as shown in Figure 2. The same boosted buck mode circuit can be used in Figure 2, but only one matrix LED driver controls the brightness of both strings. This demonstrates how each NMOS shunt power switch inside the IC can be configured independently of other power switches, allowing for a variety of matrix designs. All channel mode and single channel mode IC commands have flickerless PWM and fade in function The IC instruction set includes 1, 2, and 3 word commands. These commands are sent through the serial data line (SDA) at up to 400 kHz with the master generation clock line (SCL). The master microcontroller sends all channel mode (ACM) or single channel mode (SCM) write commands to control the brightness, fade-in, open-circuit threshold, and short-circuit thresholds for the LED channel and LT3965 addresses. Broadcast Mode (BCM), ACM, and SCM read commands require the driver to report the contents of its registers, including open and short registers for troubleshooting. When a new fault occurs, the matrix LED driver sets the ALERT flag. Microdevices can respond to a fault by determining which driver reported the fault and the type and channel of the fault. In the event that multiple ICs report a fault, they can queue the fault report to the master device to prevent overlap errors. This makes the alarm response system reliable and decisive. A complete list of registers and command sets is given in the data sheet. The ACM write command immediately uses two IC words to turn all eight channels of a single LT3965 address on or off - the channels are turned on or off at the same time. Turning on or off a large number of LEDs will provide a significant current and voltage load step to the DC/DC converter. The converter described here handles these transients very well, with little output capacitance and high bandwidth. As shown in Figure 8, writing a large number of LEDs through ACM writes does not produce visible flicker or apparent transients in the LED currents of other channels. The high-bandwidth buck mode converter built around the LT3797 is responsible for such small transient control. Figure 8: The LED matrix driver design shown here has minimal or no cross-channel transient effects. For example, transitioning half the channel - here, turning both on and turning off two channels - has little transient effect on the other four untouched channels. Unconverted channels remain flash-free Single-channel mode writes produce relatively small and fast single LED transients. The MCU write is used to turn on, off, or PWM dimming with or without fade-in function to set the brightness of only one channel at a time. PWM dimming values ​​are written in 3 words between 1/256 and 255/256, while ON and OFF can be transmitted in shorter 2 word commands. The fade-in bit in a single SCM write command enables the LT3965 to move between two PWM dimming levels with internally determined logarithmic attenuation and without additional IC traffic. Use the SCM write command to set the open and short-term thresholds for each channel between 1 and 4 LEDs. Short-circuit and open-LED fault protection for each channel Short-circuit and open-circuit protection are inherent advantages of this matrix dimmer. The NMOS power switch for each channel can be shunted between 1 and 4 LEDs in series. Traditional LED strings prevent the entire string from opening or shorting. Only some ICs have output diagnostic flags to indicate these fault conditions. In contrast, this driver prevents a single channel from being shorted and opened, thereby maintaining the operation and operation of the operating channel while recording and reporting fault conditions. When a fault occurs within a string, the driver detects the fault and sets its ALERT flag to indicate to the microcontroller that there is a problem that needs to be resolved. If the fault is an open circuit, the driver will automatically turn on its corresponding NMOS power switch and bypass the fault LED until full diagnosis or until the fault is removed. The IC saves open and short fault registers for each channel and returns data to the microcontroller during an IC fault read command. The command set includes reading of the read and clear fault registers that keeps the status register unchanged, allowing user-programmable fault diagnosis. Registers can be read in various modes that allow writing, SCM, ACM, BCM: Single Channel Mode (SCM) reads return open and short circuit register bits for a single channel. SCM read also checks open and short-term threshold registers, mode control, and 8-bit PWM dimming values ​​for this channel All channel mode (ACM) reads return the open and short register bits of all channels at a given address without clearing the bit, and the ACM ON and OFF bits for all eight channels In more complex systems with many matrix dimmers sharing the same bus, broadcast mode (BCM) reads first ask which, if any, IC address declares the fault flag ACM and SCM reads can be used to check for and clear faults and read all registers for a powerful IC communication system Up to 16 drivers can be addressed on the same bus Each IC has four user-selectable address bits that enable 16 unique bus addresses. Each ACM and SCM IC command is sent to the shared communications bus, but only the addressed IC will take action. All ICs on the bus follow BCM commands. The 4-bit address architecture allows up to 8 individual microcontrollers and a single IC dual-wire communication bus. 16 = 128 independently controllable channels. With this drive, all individual LEDs in car headlights, taillights, and decorative lights, in addition to the most ambitious lighting displays, can be controlled via a single IC communication bus and a single microcontroller. Considering that up to four LEDs can be connected per channel, a relatively easy-to-implement system can support matrix dimming for up to 512 LEDs. in conclusion Matrix LED dimmers control eight LED brightness channels on a single LED string, providing lighting designers with unlimited access to sophisticated and eye-catching automotive lighting designs. The IC communication interface allows the microprocessor to control the brightness of individual LEDs in the string. Fault protection in the IC interface ensures the robustness of the LED lighting system. Matrix dimmer channels are versatile: each channel can control multiple LEDs; channels can be used in combination to support higher current LEDs; or up to 16 matrix dimmer ICs can be used on the same communication bus A system that produces a high number of LEDs. Now is the next step in designing car headlights, taillights, fronts, sides, dashboards, and decorative lights. Shenzhen Kingwire Electronics Co., Ltd. , https://www.kingwires.com
